feat(studio,core): mute groups, and hear-only-this that cannot reach the export

B5: mute and solo, on groups and tracks (track mute already shipped by A2 —
nothing to build there).

Group mute — persisted as data-hidden on the <hf-audio-group> element itself
(never written onto members, per design doc §2.1's state-restoration
warning). Studio action reuses B7's generic setAudioGroupAttribute
(setQuiet/setLive split) rather than duplicating toggleTimelineTrackHidden's
shape — same one-atomic-patch/one-undo-entry contract, already built for
exactly this purpose. Render: B4 already drops every member of a
data-hidden group (confirmed by a new audioMixer.test.ts case — no
production change needed there). Preview: a dedicated muteGain node
(groupInput -> [fx] -> muteGain -> output -> master) so a mute toggle
never fights scheduleVolumeLane's ramps on the same param — the same
hazard B7's volume fader was split out to avoid. Mid-playback toggles
sync via a new syncAudioGroupMute pass in init.ts (a group carries no
data-start, so it's invisible to the existing visibility-node query).
Members of a muted group render the strikethrough label treatment
(TimelineTrackPlainHeader's isGroupMuted, sourced from
TimelineElement.audioGroupHidden) — display only, no attribute touched.

Solo — "Hear only this": a new session-only store slice (audioSoloSlice,
soloed: ReadonlySet<string> of clip/group ids, never track numbers, never
serialized). Predicate (isAudibleUnderSolo, packages/core/src/audioGroups.ts
so both the store and the preview transport share one definition): an
element is audible while any solo is active only if it or its own group is
soloed. "Siblings, never ancestors" lives in the graph, not the predicate —
solo gain is a per-element stage only; group buses are never attenuated by
solo, so a soloed member's path through its group stays open by
construction. Preview: a dedicated per-element soloGain in
webAudioTransport.ts (parallel to the mute mechanics), pushed via
window.__hf.setAudioSolo — a direct call, not an attribute write, so it
can't ride the visibility-diff path mute uses. media.ts's HTMLMedia
fallback folds the same predicate into its per-tick volume computation
(the same seam A2 used for data-hidden). Half-lit group indicator
(isGroupHalfLitUnderSolo) for "not soloed itself, but a member is".
Exclusive-by-default toggle, ⌘/Ctrl-click to add/remove, TimelineSoloButton
(⌗) beside mute on both track and group headers. Transport-bar banner
("Hearing only <label> — your export is not affected", Clear button) added
in PlayerControls.tsx, reading labels straight off the live preview DOM.

Export-safety, the most important property here: toggling/adding/clearing
solo never calls setAttribute/removeAttribute on any element and never
invokes the project save path (both asserted directly via spies in
audioSoloSlice.test.ts) — solo cannot reach an export by construction, not
by convention.

Also: extracted useHydrateActiveCompPathFromUrl out of App.tsx (a
pre-existing, unrelated effect) to stay under the 600-line filesize cap
after wiring useAudioSoloBridge in; and fixed a circular dependency the
solo-banner wiring introduced (useAudioSoloBridge.ts now imports
usePlayerStore from its concrete module instead of the player/ barrel,
which re-exports PlayerControls.tsx — the barrel path is what closed the
cycle).

Gates: bun run build clean; packages/core full suite 2379/2379; packages/
studio full suite 4276/4294 (18 pre-existing todo); packages/engine
audioMixer.grouping.test.ts 5/5; oxfmt/oxlint clean on all 23 touched
files; fallow clean (0 new circular deps, 0 new filesize/complexity
findings).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/studio/src/player/lib/timelineDOM.ts

@@ -70,24 +70,27 @@ function resolveClipTag(clip: ClipManifestClip): string {
 
 // One `<hf-audio-group>` scan per document, not per clip — resolveAudioGroups
 // walks the whole tree, and a parse touches every clip in it.
-const groupInfoCache = new WeakMap<Document, Map<string, { label: string; volume: number }>>();
+const groupInfoCache = new WeakMap<
+  Document,
+  Map<string, { label: string; volume: number; hidden: boolean }>
+>();
 
 function groupInfoFor(
   doc: Document | null | undefined,
   groupId: string,
-): { label: string; volume: number } {
-  if (!doc) return { label: groupId, volume: 1 };
+): { label: string; volume: number; hidden: boolean } {
+  if (!doc) return { label: groupId, volume: 1, hidden: false };
   let info = groupInfoCache.get(doc);
   if (!info) {
     info = new Map(
       resolveAudioGroups(doc).map((group) => [
         group.id,
-        { label: group.label, volume: group.volume },
+        { label: group.label, volume: group.volume, hidden: group.hidden },
       ]),
     );
     groupInfoCache.set(doc, info);
   }
-  return info.get(groupId) ?? { label: groupId, volume: 1 };
+  return info.get(groupId) ?? { label: groupId, volume: 1, hidden: false };
 }
 
 // fallow-ignore-next-line complexity
@@ -167,6 +170,7 @@ export function createTimelineElementFromManifestClip(params: {
       const info = groupInfoFor(doc ?? hostEl.ownerDocument, audioGroup);
       entry.audioGroupLabel = info.label;
       entry.audioGroupVolume = info.volume;
+      entry.audioGroupHidden = info.hidden;
     }
     const fxChain = hostEl.getAttribute("data-fx-chain");
     if (fxChain) entry.fxChain = fxChain;
@@ -392,6 +396,7 @@ export function parseTimelineFromDOM(doc: Document, rootDuration: number): Timel
       const domGroupInfo = groupInfoFor(doc, domAudioGroup);
       entry.audioGroupLabel = domGroupInfo.label;
       entry.audioGroupVolume = domGroupInfo.volume;
+      entry.audioGroupHidden = domGroupInfo.hidden;
     }
 
     // Sub-compositions